1. Field of the Invention
The present invention relates to a fuel injection control apparatus for an internal combustion engine, and particularly to a fuel injection control apparatus preferred for fuel injection of a direct cylinder injection type of an internal combustion engine.
2. Background Art
An internal combustion engine is provided with a fuel injection control apparatus which controls an injector. The fuel injection control apparatus calculates an appropriate fuel injection quantity in accordance with an operation state of the internal combustion engine and drives and controls an injector which supplies fuel. An injector is of an electromagnetic type which supplies a current through an electromagnetic coil (solenoid) contained in the injector in order to open the injector and keep a valve opening state. This generates an electromagnetic force by which opening/closing operation of a plunger which constitutes an injector is performed to inject an amount of fuel corresponding to a valve opening period.
The amount of fuel injected is mainly determined by a pressure difference between fuel pressure and ambient pressure at the nozzle hole portion of the injector, and the time in which the plunger is kept open and the fuel is injected.
Therefore, in order to perform injection of an appropriate amount of fuel, not only time to keep valve opening of the injector according to fuel pressure needs to be set, but also opening/closing operation of a plunger needs to be performed quickly and with accuracy.
However, before a plunger actually closes after energization to the injector is finished, a closing operation of the plunger is accompanied by a delay due to a response delay of a current circuit or the like. Conventionally, it is a common practice to set an electric energization time to the injector (to make a correction to an injection pulse operation in advance, as an invalid pulse width) considering the response delay.
Alternatively, a method for controlling a response delay of a current circuit to the minimum by rapidly discharging a valve opening current when a current supplied to a solenoid is switched from a high current for causing the injector to perform valve opening operation (hereinafter referred to as an opening current) to a low current for maintaining a valve opening state (hereinafter referred to as a holding current)(see Patent Document 1, JP Patent No. 3562125 publication, for example) has been known.
In addition, in order to strengthen a valve opening force of the injector in accordance with a fuel pressure, a fuel injection control apparatus which variably sets a period of time to supply a peak current when an opening current reaches a peak has been proposed. This fuel injection control apparatus shortens a peak current holding time of the opening current, when a pulse width of the injector is short. A response delay of a current circuit is stabilized by making a flowing current to the injector become a holding current when the supply of peak current to the injector ends (see Patent Documents 2, JP Patent Publication (Kokai) No. 2003-65129 and Patent Document 3, JP Patent No. 3768723 publication, for example).
In addition to them, a fuel injection control apparatus which searches a dead time correction table based on fuel pressure and corrects an invalid pulse width based on the search result has also been proposed (see Patent Document 4, JP Patent Publication (Kokai) No. 2001-164961, for example).